This application claims the priority of German patent document 10 2004 046 912.1, filed Sep. 28, 2004, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for simulation and assessment of the life or dynamic strength of components of a vehicle by means of a finite element analysis. In this process, the finite element models of the individual components are collated at interfaces, for the simulation. Forces are applied externally to the vehicle at these interfaces or at specific points within the components of the vehicle in order to determine the weakest points in the overall model with respect to dynamic strength.
Methods for computer simulation are advantageous because they shorten the development procedures for vehicles, especially for motor vehicles. Particularly in the case of crash simulation, they reduce the development costs. As a result of continuously increasing computer performance, the static and dynamic strength of components can now be calculated well by means of a finite element method (FEM). The known finite element networks can be used to model components completely and in detail, making it possible, in particular, to determine the stress distribution, in bearings or at interfaces between the components.
German patent document DE 199 24 207 A1 discloses a method for testing the dynamic strength of fuel tanks. In this case, the behavior of the tank is investigated when external forces are applied, such as those which occur in the event of accidents. This is done using a computer simulation based on finite element analysis extended by methods from multiple body simulation and contact mechanics. Since the thickness of the tank wall is highly dependent on the tank location, depending on the production, the FEM is formed from elements of different thickness. This accident simulation differs from the live simulation, however, in that a destructive force is introduced to the fuel tank only once in the crash simulation.
One the other hand, during live simulation, external forces are introduced to the vehicle at different frequencies over a lengthy period, in order to check the strength of a vehicle during continuous operation. Frequencies which are normally used for dynamic strength testing (relative to the forces and moments which are introduced to a vehicle model or model element externally) are, for example, in the range from 0.08 to 50 Hz.
German patent document DE 102 22 700 A1 discloses an optimization method for a crash simulator, which uses finite element methods to simulate the vehicle or parts of it. The dimensions of the metal sheets which are used for the bodywork are calculated on the basis of the crash simulation. In this crash simulator, an approximation model is matched to the real vehicle by optimization of a set of parameters so as to satisfy a termination condition. A plurality of iteration methods and equations are proposed for the approximation model for the crash simulation, in order to generate optimal parameters. Since elastic structures are irrelevant in this case, this method is not suitable for live simulation.
German patent document DE 100 23 377 C2 discloses a method for increasing the performance of a computer facility for finite element simulation. In this case, a numerical calculation method has been developed further in order to make it possible to carry out simulations such as these efficiently. However, the specific application of such simulators for vehicles is irrelevant.
One object of the present invention is to provide a method for simulation and assessment of the live or dynamic strength of components of a vehicle, which provides an optimized vehicle model in order to achieve an optimum simulation of the vehicle dynamic strength.
This and other objects and advantages are achieved by the method according to the invention, in which the vehicle model or model element is matched to the real vehicle in a plurality of iteration steps by introducing external forces that are selected not only on the vehicle model or model element but also on the real vehicle. The forces which occur at the interfaces or at specific points within the components on the vehicle are compared by means of elastokinematic measurements for the entire vehicle model or model element, and the elastokinematic characteristics of the various elastic structures in the vehicle model or model element are then adapted, such that the vehicle model or model element behaves like the real vehicle at the interfaces or at the specific points within the components with respect to forces/time or movement/time. The iteration steps are repeated in order at the same time to adapt the elastokinematic characteristics optimally for a plurality of interfaces or a plurality of points within the components.
According to the invention, it has been determined that the dynamic strength test carried out so far on the real vehicle can also be carried out by means of a finite simulation. In contrast to the known crash simulations, the vehicle model is subjected to a set of simulated dynamic strength test runs rather than having a destructive crash impulse applied to it. In this case, by way of example, an uneven route is recorded, and its forces and moments acting on the vehicle are detected.
The same dynamic strength driving profiles as already applied to the real vehicle during dynamic strength testing are then applied to the vehicle model or model element in the simulation for dynamic strength testing over a predetermined time period of, for example weeks or days. Forces and moments in a frequency range from, for example, 0.08 to 50 Hertz are introduced to the vehicle model or model element during dynamic strength testing. These frequencies correspond to those which are most damaging to the life or dynamic strength of the real vehicle, and its materials.
In contrast to a crash simulation, the elastic structure of the vehicle chassis, engine suspension or running gear is of major importance when using finite element simulation for dynamic strength testing. As a consequence, the vehicle model or model element must be matched precisely to the real model before the dynamic strength simulation. However, this procedure itself has yielded problems in dynamic strength simulations such as these. According to the invention, an iterative method is now provided for the simulation, in which the parameters for the vehicle model or model element are adapted on the basis of the static finite element model, and the elastokinematic characteristics for the elastic structures are optimized to an ever greater extent in further iteration steps. As a result, the vehicle model or model element is optimally matched to the real vehicle for the simulation.
The elastokinematic characteristics are, for example, the inward-springing characteristic or damper characteristic of the running gear. On the other hand, spring constants and damping characteristics can also be modeled for rubber buffers or other elastic materials. The flexibility of the materials used for components of the vehicle can also be modeled in this way. A vehicle model or model element which has been optimized in this way can be used in the simulation in order to carry out a life test on the individual components of the vehicle on computer systems. This makes it possible to draw conclusions regarding the life of the components, without the need to perform work on test rigs over periods of months, on the real vehicle. Instead of modeling of the entire vehicle, it is also possible to use a vehicle model element (for example, only the running gear), the chassis, the internal fittings or the engine. The forces which act as internal sectional loads on the running gear subsystem are then modeled as external forces, while the dynamic strength test patterns, in the form of road unevenness data, are modeled as an external force from underneath against the running gear.
In conjunction with the method for simulation, provision is made for the vehicle (or parts of it) to be modeled using the finite element method, and for an evaluation algorithm to be carried out in a further step after calculation of the forces and moments, with the evaluation algorithm assessing the life and the dynamic strength of the individual vehicle parts. By way of example, this assessment calculates the weakest point in the investigated vehicle model or model element; and, with optimum matching, leads to the same results as a test on the real test rig with the real vehicles. In both cases, potential damage points are determined, for example, on the chassis and at interfaces between the engine, transmission and vehicle bodywork.
This is referred to as a hybrid method for determination of the dynamic strength. It is subdivided into a step for determination of the dynamic overall range of vehicle sectional loads using a multiple body simulation, and a second step of subsequent dynamic strength analysis by means of a software program system.
This results in statements about the reliability of specific components of the vehicle and, in turn a calculation of the damage distribution on the bodywork of the vehicle. The method is particularly suitable for checking damage at weld points at interfaces between individual components. A further potential results from the virtual matching to a real test rig for life determination on the vehicle, making it possible to determine the movements during operation of the individual operating pistons of the real simulator in order to produce specific forces on the vehicle. These values can be used particularly well as manipulated variables for the movements during operation for testing of real components on the vehicle or vehicle subsystem. These manipulated variables for the movements during operation can also be used for multiple component simulation or FEM calculation for life determination.
The described method can be used to simulate dynamic strength during operation of the vehicle even in response to stochastic loads (for example, on poor road surfaces), and their effects on the life of the components. The calculational assessment makes it possible to shorten considerably the time required for life test processes in comparison to the real tests. The simulation makes it possible to save vehicles which are loaded until they become damaged during long-term driving trials on roads and test tracks, or else on a test rig, thus obviating the need to subsequently scrap the vehicle. The aim of the method according to the invention is to improve qualitatively the numerical life prediction by means of a virtual test rig. In particular, the virtual life test rig is matched to the real vehicle by an iterative adaptation process, for this purpose.
The method for simulation and assessment of life and operating strength according to the invention can be matched either to the real vehicle or to the real vehicle test rig, thus resulting in an identical load situation on the vehicle bodywork on the real bodywork test system. The life simulation process carried out by calculation is quicker and more detailed than that on the real bodywork test system. Finite element modeling results in a digital map of the real vehicle prototypes, on demand. The convergence between the results with the digital prototype and the real prototype becomes ever better by means of the simulation according to the invention over the development time period, with elastic structures being adapted by changing parameters such that the real vehicle model is optimally simulated. Since the finite element models for individual vehicle components are stored in libraries and can thus be used repeatedly, it is also possible to carry out reliable comparisons for planned vehicle variants, even in the development stage.
The vehicle model or model element includes elastic structures for optimum modeling of the real non-linearities between the individual components. In this case, during the multiple body simulation, the elastic structures are created at the interfaces between the components which are modeled by the finite element method. Since the inclusion of the elastic structures in the finite element models requires additional computation time, the elastic structures are preferably created and matched to the interfaces in such a way that the various elastic effects are implemented in a generalizing form in the elastic structure at the interface. For this reason, the vehicle model or model element must be matched to the real vehicle by means of a plurality of iteration steps using the iteration process according to the invention. For this purpose, the elastic structures are first reduced by calculation to rigid structures of the simulation, by setting the elasticity to an infinite value.
The static vehicle model produced in this way is first of all roughly matched to the real circumstances. Thereafter, the elasticities are then adapted by means of a numerical method, so that the vehicle model is optimized for the simulation. Various test data records (for example, road data, driving over curb stones and the like) are available to the simulator for this purpose. Since the real vehicle and the real test rig with the vehicle arranged on it were tested using the same road data, the reactions of the vehicle structure and the forces on the real interfaces are known. The simulated vehicle model is now matched to the real movement and force profiles in the real elastic structures. For this purpose, a plurality of test data records are offered to the simulator, with the elasticity parameters of the elastic structures being changed on each occasion, so that the vehicle model converges with the real structures. Correlation investigations are carried out using standardized test rig models. These investigations are carried out in the quasi-static range of elastokinematic at very low stimulation frequencies of the vehicle model, and in the dynamic range with the aid of deterministic, real and virtual trial runs with the vehicle. The measurement results from the real measurements on the vehicle can thus be used optimally for the iteration of the vehicle model.
The elastic structures are adapted, for example, using parameters such as masses, coefficients of inertia, stiffness characteristics etc., which, for example, have been predetermined on the basis of a real rubber bearing or a real suspension leg, and which are still being adapted during the simulation. In this case, spring characteristics can be adapted very well by the adaptation of the elastic structures in quasi-steady-state calculations. Elastic structures can simulate the elasticities of the front and rear axle structure or the circumstances at wheel hubs and shock-absorber units.
Once the rigid vehicle model has been calculated thoroughly in the first step, the reaction of the vehicle bodywork for the dynamic load situation can be measured with simultaneous inward springing or with vertical or horizontal dynamic load situations, on the basis of which the mass, inertia and damping characteristics of the model can then be adapted. The vehicle model and model element is then validated in a final iteration step. For this purpose, a real motor vehicle (or a ship or an aircraft) which is fitted specifically with measurement sensors, is checked for the dynamic strength loads by means of dynamic stimulation. The results obtained in this way are also used to adapt the coefficients for the life or dynamic strength calculations for the simulator, so that the life and dynamic strength calculations approximate reality.
The following points must be unambiguously defined for the life calculation. The development state of the vehicle on the test rig and its components must be documented with respect to the parameters. The transmission response of all the relevant components, such as the suspension/shock absorbers and elastomer bearings must be recorded. If necessary, complex components must be remeasured in their static and dynamic operating ranges. The results obtained in this way can be stored in a digital database and are likewise available for further life investigations for other vehicle types.
The cylinder movements introduced into the running gear must be defined unambiguously in terms of the magnitude and direction in order to simulate the dynamic load situation on a poor road surface for the simulation. The stimulation while driving over a poor road surface is produced both in the real test rig and in the simulation model by means of test cylinders via which the test rig is connected to the traffic model. The predetermined deflections of these test cylinders act on the vehicle as if it were traveling over a poor road surface. For this purpose, the test cylinders must apply forces to the vehicle model. On the other hand it is also possible to introduce torques (for example, to the wheel suspension on the vehicle model) by means of specific test cylinders which can rotate. Finally, the static load distribution within the vehicle must also be measured so that any prestresses occurring at specific interfaces and in the area of the elastic structures can be calculated correctly.
According to another embodiment of the invention, the forces and moments are introduced into the vehicle via simulated movement controllers which are clamped in between the fixed foundation and the interfaces and operate in a translational or rotational direction with respect to the interfaces of the vehicle model element or model. The movement distances are calculated from the simulation of the movement distances required for the movement controllers, in order to apply the real force and/or moment load to the vehicle model or model element.
The method for simulation and assessment of the life and dynamic strength can also be used, according to the invention, to calculate the movement distances of the movement controller (such as the test cylinders). By way of example, the movement distances for the movement controllers for a force and moment load on the vehicle can be produced in order to provide a comparative simulation on driving over an uneven road. On the real test rig, an uneven road is simulated by hydraulic test cylinders producing movement stimuli in one or more directions statically and dynamically from the exterior, acting on the vehicle running gear. These impacts are achieved by extending the movement controllers in the form of an impulse. Different movement distances are required for the simulation depending on the characteristic of the movement controllers, for example movement pressure and the diameter of the cylinders.
In order to reduce this complex process for the real test rig, the computer simulation method can calculate the required forces on the basis of driving over this uneven road, and can calculate the movement distances for the movement controllers. For this purpose, the method for simulation of the life or dynamic strength calculates the necessary movement distances for this purpose from the movement controller geometry and from the drive pressures for movement controllers (for example, the hydraulic pressure or the current for electric motors), in order to simulate precisely and exactly the same movement distance with respect to the force and moments as that which would occur when driving the real vehicle over this uneven route. In this way, the method for simulation and assessment can be used not only for calculation of the life and dynamic strength but also for designing the real test rig for the vehicle or vehicle subsystem.
In one preferred embodiment of the invention, the position of the vehicle model or model element is defined in space by defining the wheel holder interfaces at the movement controllers. One problem that arises in a virtual simulation system is that the vehicle model becomes desynchronized when forces and moments are applied to the running gear, and departs from the simulation layout in an uncontrolled manner. Accordingly, for this purpose, the vehicle is fixed on the real test rig in a moving manner (for example, in the area of the bumpers and wheel holder interfaces); that is, it is restrained. Since the simulation is an unrestrained configuration with external movement stimulus, the wheel hub forces are produced as a reaction during the simulation. However, in order to prevent uncontrolled effects in the simulation in this case, the vehicle model is fixed, by calculation, on the movement controllers in the area of the wheel holding interfaces.
The method for simulation is designed such that horizontal deceleration and/or acceleration forces can be introduced to the vehicle model. For this purpose, the movement controllers introduce deceleration and acceleration forces in the horizontal direction into the vehicle model. In this case, the movement controllers are attached to the front structure and in the rear area of the bumper in the horizontal direction, and the forces and moments are calculated there.
The invention provides a virtual test rig which can be used for dynamic strength testing or live simulation for the overall vehicle by numerical simulation. Such simulation is used in particular during vehicle development to identify weaknesses in its design, even in this early phase. For this purpose, dynamic strength simulations or real test results from previous development phases with other vehicles can be used by storing this data in a library or in a digital mock-up system. The virtual test rig iteration according to the invention simulates the force/moment situation on the vehicle, and it is thus possible to calculate the internal forces between the individual components within the vehicle. The measures developed from this result in increased maturity in the development of new complex vehicle types.
The invention also provides software program products which include the assessment method for simulation of the life and dynamic strength of components according to the present method. Software program products such as these include floppy disks, memory chips and entire computer systems with program elements of the method for simulation.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.